A large number of industrial applications for rotating electrical machinery call for efficient variable speed motor drive systems of 100 horsepower or more. Such applications include motor drives for large pumps and high volume fans. Traditionally, such applications use DC motor drives because of the variable speed motor requirement.
The fairly recent availability of reliable high-power solid-state electronic components has made feasible the construction of inverters for driving variable-voltage, variable-frequency AC motor drives. The development of competitively priced large AC motor drives has been hindered by the fact that the least expensive type of inverter, the line-commutated inverter, and the least expensive AC motor, the squirrel-cage induction motor, are functionally incompatible in terms of their reactive power requirements.
In AC circuits the current and voltage waveforms are not always in phase. The power factor of the circuit is the cosine of the phase angle between the voltage waveform and the current waveform. When the phase of the current lags or leads the voltage, the corresponding power factor is referred to as a lagging power factor or a leading power factor, respectively.
For high power inverter applications, the most suitable solid-state switching device is the thyristor, which becomes conductive when a low power level signal is supplied to its gate terminal, and remains in a conductive state until the current through the thyristor is reduced to a near-zero value. At this point, a reverse voltage polarity across the main terminals of the thyristor is required for a short time interval to prevent the device from reverting to a conducting state. The process of turning off the thyristor by supplying this reverse voltage polarity is commonly referred to as commutation of the inverter.
A forced-commutated inverter requires appropriately charged energy storage devices which can be discharged across the thyristor to turn it off. Forced-commutated inverters typically need extra components such as additional thyristors, diodes, capacitors and inductors to turn off the thyristor. The additional circuitry required by a forced-commutated inverter makes the cost of the inverter approximately twice that of a line-commutated inverter, which does not require additional circuitry for turning off the thyristors. In a line-commutated inverter a sinusoidal power supply provides the voltage source that forces the thyristors in the inverter off in a sequential manner.
AC induction motors typically operate at a lagging power factor, while the more expensive AC synchronous motors may operate at a leading power factor. Of the two types of inverters mentioned above, the line-commutated inverter typically supplies motor loads which operate at a leading power factor, and the forced-commutated inverter supplies motor loads which operate at a lagging power factor. It is therefore evident that the most desirable combination, namely that of a line-commutated inverter driving an AC induction motor, is not practical, particularly in applications which require the motor to operate at various speeds.
Thus, two AC motor-drive systems have been available for applications requiring a high power variable speed motor drive The first, which uses a synchronous motor with a line-commutated inverter, simply cannot compete with the cheaper DC motor drive systems. The second alternative, that of driving an induction motor with a forced-commutated inverter is also not cost-competitive with DC motor drives. In addition, the second alternative is difficult to implement in very large motor drives, due to the large currents which must be forced-commutated. Neither of the aforementioned AC motor-drive system has seen substantial success in competing with DC motors for use in applications requiring large, variable speed motor drives.
Examples of using an AC induction motor with a line-commutated inverter may be found in U.S. patent application Ser. No. 331,108, entitled "Leading Power Factor Induction Motor Drive", filed Dec. 15, 1981 by Gabor Kalman and Graham W. McLean, and U.S. patent application Ser. No. 374,375, entitled "Variable Speed Induction Motor Drive System" filed May 3, 1982 by Gabor Kalman. These applications, both of which are assigned to the assignee of the present invention, solve the fundamental incompatability between the line-commutated inverter and the induction motor by providing an auxiliary rotor to supply the reactive power requirements of the main machine. While such applications work quite well with small machines, it is desirable to have a variable speed induction motor drive system which is capable of operating with large induction motors of a standard configuration, that is, motors not including the auxiliary machine.